Theoretical proposals followed by various experimental measurements have been carried out to address these two questions.
Classified by the irreducible representations of the point group \(D_{3d}\) , the possible pairing symmetry of Bi(Te,Se) systems has been theoretically proposed as \(A_{1g}\)  phase (\(s\) -wave), odd-parity \(A_{1u}\)  phase[1]}, [2]}, [3]} or two-component \(E_u\)  phase (nematic[4]}, [5]} versus chiral[6]}). For doped topological insulators, some experimental data support the \(A_{1g}\)  phase (\(s\) -wave)[7]} without topological superconductivity[8]} for Cu-doped Bi\(_2\) Se\(_3\) , while others support the nematic \(E_u\)  phase for Bi\(_2\) Se\(_3\)  doped with Cu[9]}, [10]}, [11]}, Sr[12]}, [13]}, [14]}, [15]} or Nb[16]}, [17]}, [18]}, [19]}, [20]}, which are believed to host topological superconductivity, although discrepancy still remains[20]}, [5]}.
For topological insulators under pressure, experimental data indicate unconventional superconductivity in Bi\(_2\) Se\(_3\)[23]}, while no direct evidence on unconventional superconductivity has been provided in Bi\(_2\) Te\(_3\)[24]}. The issue regarding whether topological superconductivity can take place in topological insulators has not been settled yet but is highly desired from the science community. To address this issue, we conduct a careful experimental and theoretical study on both the normal phase and superconducting phase of Bi\(_{2}\) Te\(_{2.7}\) Se\(_{0.3}\)  at high pressures. The details are given as follows.
